If you've received the OnePlus One XNPH30O OTA update, you might have noticed your battery life was severely reduced since installing it. You're not the only one - the CyanogenMod issue tracker for the One has a thread with well over 100 comments on the subject at this point.
CM seems to have nailed down the issue to problems with the power management and the proximity sensor. The former issue has been fixed and will be likely live in nightly builds starting today, and an OTA is being rushed through for those only using stable builds. The proximity sensor issue hasn't been resolved at this point, because CM is having a difficult time reproducing the issue reliably.
Steve Kondik alleges the proximity problem is the result of a race condition that causes the phone's sensor to enter a "bad" state which then prevents the device from fully suspending (eg, ending wakelock). You can see all of Steve's comments on the thread here, here, and here.